1. Field of the Invention
This invention relates to an improved container for growing plants. More specifically, but not by way of limitation, the invention relates to an apparatus for root-pruning plants grown in containers.
2. Discussion
Today, nurseries typically grow landscape plants and other plants intended to be transplanted outdoors in containers above ground. Such container-grown plants can be grown at a faster rate than those grown in the field, and are easier to handle and display. In addition, because the roots of container-grown plants are not severed or otherwise disturbed when the plants are transplanted, transplanting can occur at any time during the year, not just during the early spring as required with bare root or bagged nursery stock.
When a germinating seed grows in a common container, typically a tap root forms first and quickly grows to the bottom of the container where it changes direction and continues to grow. If the root somehow becomes physically trapped or exits the container and is air-pruned, the tap root growth is terminated, and as a result, the apical dominance of the root tip is lost and the development of secondary lateral roots along the length of the tap root occurs. The lateral roots generally grow outwardly towards the sides of the container and slightly downwardly due to geotropism. When the lateral roots contact the sides of a round container, they circle the sides toward the bottom of the container where they continue growing in a circular pattern, sometimes for four or five more revolutions.
When a plant having root development of the type described above is transplanted, the "spiral" root development and generally downward orientation of the roots provides little or no lateral anchorage for the plant, and the root tips are at a depth in the soil where aeration is less favorable. In addition, the downwardly oriented roots are intermingled with each other and as such roots increase in diameter with age, they exert pressure on one another which restricts the normal flow of water and nutrients from the root tips to the top of the plant as well as the downward flow of energy from the leaves.
It has been shown that the root growth of container-grown plants after being transplanted primarily involves extensions of roots which were present in the container at the time of transplanting as opposed to the development of new roots. Consequently, the number and the position of root tips present at the time of transplanting is important to the rapid establishment, and frequently, the survival of container-grown plants. Thus, it is desirable for the plant to have roots poised to grow in all directions once it is planted in the landscape. This increases the rate of water and nutrient absorption plus provides secure root anchorage to keep the plant upright.
To address these concerns, landscape plants have been grown in bottomless containers placed on wire surfaces whereby roots reaching the bottom of the container are air-pruned. The air-pruning causes the root to branch, much like cutting off the tip of a twig or branch on the above-ground portion of a tree or shrub. However, plants grown in bottomless containers still end up with most of the root tips produced at the bottom of the container, and the requirement of placing the containers on wire screen surfaces generally makes the practice uneconomical. Further, such open bottom containers are not easily moved without spilling or losing some of the growth medium from the container.
More recently, containers have been developed for controlling spiral root growth whereby the root tips are physically trapped by surfaces in the container and are prevented from elongating, or the circling root tips are caused to pass through openings in the sides of the container whereby the roots are air-pruned. An example of the root trapping type of container is described in U.S. Pat. No. 4,442,628 issued Apr. 17, 1984. An example of a container including vertical side openings for air-pruning circling roots is described in U.S. Pat. No. 4,497,132 issued Feb. 5, 1985. While such root trapping and air-root pruning containers have been utilized successfully, some spiral root growth still takes place and the development of lateral root tips has been less than optimum.
U.S. Pat. No. 4,753,037 recently disclosed a container which has proven very effective in directing roots of plants into openings throughout the side wall and bottom of the container to facilitate air-pruning. However, the slopes and angles of the container has made the making of molds and manufacture of containers of this design difficult and costly.
Thus a need exists for an improved container for growing plants intended to be transplanted which minimizes root circling and promotes root branching in a manner which is cost effective and easy to manufacture.